Aggressive behavior is a quantitative trait, affected by multiple interacting genes with individually small effects, whose expression is sensitive to the environment. I propose several complementary approaches to investigate the genetic basis of aggressive behavior in Drosophila males, using an assay that I developed to rapidly and reproducibly quantify aggressive behavior. Specifically, I will: (1) Map quantitative trait loci (QTLs) affecting variation in aggression. I have localized one QTL with a large effect and will use complementation tests to deficiencies and mutations of positional candidate genes to identify candidate gene(s) corresponding to the QTL. (2) Use artificial selection to derive lines with increased and decreased levels of aggression. This experiment is currently at generation 7.1 will assess whether other traits have evolved as correlated responses to selection for aggression, and use whole genome expression profiling to identify candidate genes whose expression differs significantly between the selection lines. (3) Identify new candidate genes regulating aggressive behavior by quantifying the aggressive behavior of a collection of single P-element insertion lines in a homozygous genetic background.